Research has been conducted in recent years on reducing the amount of reflection return light of a liquid crystal screen such as that of a television or cell phone by providing an antireflective structure having a fine irregular surface structure in which the period of the irregular surface structure is controlled to be equal to or less than the wavelength of visible light.
A method in which an antireflection article (transfer article) is manufactured by forming a fine irregular surface structure by carrying out anodic oxidation treatment on the surface of a prototype aluminum mold, obtained by casting and plastic working an aluminum alloy, and transferring the irregular surface portion to a molding material such as a resin, is employed as a method for providing an antireflective structure having a fine irregular surface structure. In addition, pyramidal shapes such as a cone or quadrangular pyramid have been reported as irregular surface patterns formed by anodic oxidation treatment.
In the case of carrying out anodic oxidation treatment on the surface of a prototype aluminum mold and manufacturing a transfer article by using this surface as a mold (stamper), since the surface of the stamper is reflected directly in the transfer article, the quality of the surface thereof is important.
Examples of factors that influence the surface quality of the stamper include “second phase particles”, “crystal grain size” and “crystal orientation” in the prototype aluminum mold as well as “mirror polishability” of the prototype aluminum mold material. Second phase particles are considered to have reciprocal properties with other factors (crystal grain size, crystal orientation and mirror polishability).
Furthermore, “second phase particles” refer to particles that form a phase that differs from the aluminum parent phase, and examples thereof include iron (Fe)- and silicon (Si)-based compounds present in aluminum alloy.
In general, if contaminants such as second phase particles are present on the surface of a prototype aluminum mold, discontinuous convex portions form during anodic oxidation treatment, and second phase particles slough off resulting in the formation of concave defects or defects occur on the surface of the stamper. If this stamper is used for transfer, the transfer portion corresponding to the defective portion of the stamper has discontinuous surface irregularities, transmitted light is scattered and antireflective properties of the transfer article decrease.
In this manner, the presence of second phase particles has an effect on the quality of the transfer article. In particular, in the case the circle equivalent diameter (to be referred to as “equivalent diameter”) of second phase particles present on the surface of a prototype aluminum mold or stamper is larger than the wavelength of visible light, the haze value of the resulting transfer article increases considerably. In addition, even in the case the equivalent diameter of second phase particles is smaller than the wavelength of visible light, if a large number of second phase particles are present, the haze value of the transfer article also ends up increasing.
Consequently, the number of second phase particles in an aluminum die that can become defects on the surface of a stamper is preferably as low as possible. In order to decrease the number of second phase particles, high purity aluminum is used that contains low levels of additional elements and impurities that give rise to second phase particles, and a stamper has been proposed that uses pure aluminum having purity of, for example, 99.99% (see, for example, Patent Document 1).
However, crystal grains of high purity aluminum easily become coarse in the casting step, plastic working step and heat treatment (annealing) step. Since macroscopically conspicuous coarse crystal grains form similar grain boundary patterns in oxide films as well and these patterns are also transferred to the transfer article by the stamper, the appearance of the transfer article is impaired.
Therefore, although a cast structure can be refined by adding a refining agent (such as an aluminum-titanium-boron-based (Al—Ti—Bo-based alloy)) to the molten metal immediately before casting, in the case of high purity aluminum, this refining becomes difficult and the amount of refining agent added tends to increase. As a result, second phase particles attributable to additives (such as titanium diboride (TiB2)) increase and surface quality of the stamper is impaired.
A prototype aluminum mold for stampers has been proposed for use as a prototype aluminum mold for solving these problems that inhibits the appearance (deposition) of second phase particles by having fine, uniformly oriented crystal grains in conjunction with reducing the content of impurities as much as possible by limiting the content of magnesium (Mg) contained in the aluminum molten metal to 0.5% by mass to 3% by mass and limiting the content of elements other than Mg, including impurities, to 500 ppm or less (see, for example, Patent Document 2). According to Patent Document 2, an irregular transfer surface having a non-directional, uniform pattern can be formed by carrying out anodic oxidation treatment on this prototype aluminum mold for stampers.